Inverters are used to convert a DC voltage into an AC voltage. In particular in the area of photovoltaics, the highest possible efficiencies are required in this connection.
Solar modules for converting sunlight into electrical energy initially supply a direct current, which must be converted into an alternating current prior to being fed into the power grids of the electric utility companies. Inverters are used for this purpose as described, for example, in U.S. Pat. No. 6,392,907. The electrical energy stored in the capacitors of an intermediate circuit is output to the power grid via a series connection of multiple semiconductor switches. In so-called 3-level inverters, during the positive half wave of the power grid and for feeding in effective power, the system switches back and forth between a median voltage level and a high, positive voltage level of the intermediate circuit in order to approximate the currently required voltage for example by pulse-width modulation (PWM). During the negative half wave of the power grid, the system switches between the median and a negative voltage level of the intermediate circuit. The infeed into the power grid occurs via a power choke, by which the AC voltage produced through quick switching between the three voltage levels is smoothed. In this manner, an AC voltage suitable for an infeed is generated separately for each phase of the power grid.
U.S. Pat. No. 6,392,907, mentioned above, explains a problem occurring in such an inverter as a result of the quick switching operations. An inverter requires a great variety of diodes and semiconductor switches, which in certain operating states are in danger of being destroyed, e.g., if a current flowing in the diode is switched off too abruptly by a reversal of the polarity of the diode in the blocking direction. This is because for a brief moment following the polarity reversal a diode is conductive in the blocking direction since charge carriers are still present in the blocking zone. Because of this effect, which is also called the reverse recovery effect, it is necessary to limit the change of the current over time and thus the change of the voltage across the diode. U.S. Pat. No. 6,392,907 describes a passive circuit for performing this task.
Such a passive circuit, however, is not optimal in all operating states. Thus, currently, inverters used for feeding into power grids are also required to be able to feed in a certain proportion of reactive power in order to be able to ensure the stability of the power grids even when there is a high proportion of energy sources that feed in in a decentralized fashion. Since controlling rectifiers for feeding in reactive power differs markedly from controlling rectifiers for feeding in effective power, passive design approaches are not optimal for avoiding the reverse recovery effect even with respect to the efficiency of an inverter.